In Nigella's recipe for Vanilla Fudge, there is no temperature mentioned. To what temperature should it be cooked?
Nigella's Vanilla Fudge is a cooked fudge that is boiled to what is known as "soft ball stage". The cooking time can vary according to the size of the saucepan. If you have a sugar, candy or jam thermometer then many of these will have the various stages for sugar marked on the themometer. Soft ball stage is 112-115c (234-240c).
You can also test for soft ball stage by using a glass of cold water. When the fudge has boiled for about 10 minutes start to test by dropping a little of the mixture into a glass of cold water. If the mixture forms a thread as it sinks into the water then it needs cooking for a little longer. If the mixture sets and forms a ball that you can squish easily between your fingers then it has reached the correct stage. For an easy fudge that is not boiled then you may want to consider Nigella's Chocolate Pistachio Fudge.